Brian's right. As I work on structuring my own sci-fi epic (thread to come), I'm trying to avoid many of the problems Moore and his team ran into.
The main one being, too much of an overarching mythology that wasn't figured out beforehand.
I'm currently reading the Mistborn trilogy (Brandon Sanderson, I recommend it for fans of *hard* fantasy). It has GOBS of mythology, a backstory that's being pieced together bit by bit, but the difference is, Sanderson knew what was coming from the very beginning. Certainly, not before he ever started writing it, but before he published a single page, he knew why the world was covered in ash by day and mists by night.
I fully believe in having organic stories, in not having character arcs planned from the start, for coming up with an idea in season 4 that you had never thought of before, and inserting it. But not at the cost of a mythology you've set up. That has to stay consistant, otherwise you're going to lose the audience.
Which, by the way, the bit of Battlestar I *mainly* argue with is the revelation of the Final Five. While their retconning works for the most part, the very fact that it wasn't foreshadowed makes it not a twist, but a deus ex machina.
Remember the wise words that someone said that I'm too lazy to look up:
"All endings are deus ex machinas, the difference is that some writers go back and put in clues."